Product formed by method of controlling resistivity of plated metal

ABSTRACT

Method and product resulting therefrom of controlling the residual resistivity of an electrolessly deposited metal by first, calculating the mathematical relationship between the residual resistivity of the deposited metal and its rate of deposition and second, depositing said metal at a rate to produce a predetermined residual resistivity.

This application is a continuation of application Ser. No. 421,349,filed May 19, 1988, now abandoned which is a division of applicationSer. No. 889,661, filed Jul. 28, 1986, now U.S. Pat. No. 4,756,923,granted Jul. 12, 1988.

FIELD OF THE INVENTION

This invention relates generally to the plating of metals, and moreparticularly to control of resistivity of the metal during the platingprocess.

BACKGROUND OF THE INVENTION

Electrical resistivity of plated metals is of foremost significance andconcern in circuit fabrication, such as in the construction of printedand integrated circuits. Achievement of the lowest possible electricalresistance in the conductive current paths with accompanying circuitintegrity are among the more important and continuing goals. Thesecharacteristics can be obtained only through close monitoring andcontrol during the metal deposition processes. Successful, uniformplating capability has developed into a highly skilled art.

The resistivity of metals is influenced by many factors, includingmaterial lattice structure, impurity content and temperature. In circuitfabrication by additive plating processes, copper is one of the primarymetals used because of its relatively low resistivity and favorableductility. In additive processes, the metal is selectively deposited ona substrate by immersion in a plating bath that may be either anautocatalytic electroless bath, or an electroplating bath. Theautocatalytic bath process is slow with copper, for instance, beingdeposited at the rate of 0.002 millimeters per hour. The bathformulation and plating process are critical to plating performance andare carefully controlled to insure and maintain bath stability, platinguniformity, and desired electrical and mechanical characteristics of themetal.

Electrical resistivity, a measure of the metal's ability to conduct-anelectric current, can be separated into two components. One is the idealtemperature-dependent resistivity due to lattice vibrations, and theother is the residual resistivity due to impurities and latticeimperfections. At any given temperature, the ideal temperature-dependentresistivity is constant but the total resistance can vary amongspecimens of the same metal depending upon the magnitude of the residualresistance. It then becomes apparent that total resistivity can bechanged only by altering the residual resistivity component.

This residual resistivity can be decreased in smelted metals by furtherrefinement to improve the purity. Plated metals are not amenable to thisprocessing and their residual resistivities have not been generallyconsidered a controllable property.

OBJECTS AND SUMMARY OF THE INVENTION

It is accordingly a primary object of this invention to provide a methodof controlling the residual resistivity of a metal, that is, resistivitydue to impurities and lattice imperfections, when deposited on asubstrate.

A further important object of this invention is to provide a method ofplating metal on a substrate that permits variation of inherentproperties of the metal as it is being deposited on a substrate.

Another significant object of this invention is to provide a method ofdefining particular desired values of residual resistivity for a platedmetal and achieving those values during the plating process.

Yet another object of this invention is to provide a plated metal havingpredetermined but variable values of residual resistivity.

A still further object of this invention is to provide an improvedmethod of plating metal having a higher purity than heretofore possible.

The foregoing objects are attained in accordance with this invention bydetermining the relationship between the value of residual resistivityas a physical property of a deposited metal and the rate at which themetal was deposited, and thereafter plating the metal onto a workpieceat a rate that will produce the desired resistivity value for the metal.This method of choosing the residual resistivity of a plated metal canbe used to enable creation of a resistance profile of the metal thatdiffers across its thickness by altering the plating rate duringdeposition. Metal purity and the number of imperfections in the latticestructure change as a result of the deposition rate change and theresidual resistivity can be selected for a plated layer. The quality ofthe deposited layer can therefrom be specified or predicted. Otherproperties apparently are not adversely affected by the plating ratechanges. The invention further permits improved plating efficienciesthrough shortened processing times.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of a preferred embodiment of the invention considered inconjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are graphs of plating rates versus ratios of theresistances measured for plated copper samples, with each ratiocalculated from the resistances measured at two different temperatures,and illustrate the effect of rate changes on residual resistivity.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Additive construction of circuit conductors is frequently accomplishedby immersion of a selectively sensitized substrate in an autocatalyticor electroless plating bath. For copper, the bath will typically containconstituents such as a metal source of copper sulfate, a pH adjustersuch as sodium hydroxide, a ductility promoter such as sodium cyanide, areducing agent such as formaldehyde, a commercial wetting agent and acomplexing agent such as ethylenediamine tetra acetic acid, all held ina temperature range of 60° to 80° C. Copper atoms will deposit from thesolution onto the previously prepared substrate surfaces. The depositionrate is slow and immersion must be continued for several hours to buildthe necessary metal thicknesses. Proportionalities of bath constituentschange with use and, even with replenishment, the bath stability, rateand quality of the metal deposition are not constant. The quality ofmetal plated from autocatalytic baths has long been thought to trap thefewest impurities and provide the maximum conductivity when the platingrate was moderate permitting the bath to be maintained at its moststable condition, that is, not to suddenly plate out onto all surfacesin an uncontrolled manner.

It has been discovered, however, that both the purity and molecularstructure of the deposited metal are improved at faster plating rates.Whereas 0.002 millimeters of plated copper per hour is a typical andestablished plating rate for copper baths, it has been found that bathmetal purity and molecular lattice structure, both affecting residualresistivity, are benefited by faster deposition at up to twice thenormal rates, such as 0.003 to 0.004 nun/hr. An inverse relationshipapparently exists between the plating rate and residual resistivity; asthe deposition rate increases, the residual resistivity decreases. Thespecific relationship can differ, however, from bath to bath.

Resistivity ρ of the metal can be separated into two components asfollows:

    ρ=ρ.sub.T +ρ.sub.0                             (1)

Where ρ_(T) is the ideal temperature-dependent resistivity due to thelattice vibrations and ρ₀ is the residual resistivity due to impuritiesand lattice imperfections. At a given temperature, ρ_(T) is constant butthe total resistivity can vary depending on the magnitude of ρ₀. Thecharacteristic residual resistivity relationship of a particular platingbath can be determined by applying the so-called "RHO" tests to samplesplated under the same conditions but at different rates. RHO testing isaccomplished by measuring the D.C. resistance R of a sample of platedmetal at two different temperatures T1 and T2 and ratioing the resultsas: ##EQU1## Where T1 and T2 are usually the temperatures of ice water(273° K.) and liquid nitrogen (80° K.), respectively. The residualresistivity ρ₀ can be solved for by rearranging equation (2): ##EQU2##

It ham been determined that the value of the residual resistivity ρ₀ forautocatalytic copper plating baths decreases with increasing platingrates. This results in the value of RHO increasing in magnitude with afaster plating rate. Such a result is opposite to an intuitiveexpectation because the typical assumption is that higher rates ofelectroless metal deposition will trap or entrain more impurities andthereby increase residual, hence, total resistance.

This unusual result can be illustrated by examples:

EXAMPLE 1

An autocatalytic copper plating bath was initially maintained in thefollowing manner: the constituent concentrations affecting plating ratewere formaldehyde at 2.2 to 3.0 milliliters per liter, copper at 8.8 to9.4 grams per liter, and cyanide at 9.0 to 13.5 milliliters per liter.RHO monitors of epoxy-glass fiber substrates, each with a circuit pathsensitized by immersion in stannous chloride and palladium chloridesolutions, were immersed in the bath for 12-hour periods. This bathproduced plating rates between 0.00198 and 0.00200 mm/hr. of copperplating. Resistance was then measured using a four-point probe systemand RHO values of 6.62 to 6.67 were exhibited. Thereafter, theconstituent concentration necessary for higher plating rates wereprovided and maintained with formaldehyde at 2.4 to 3.4 ml/l, copper at9.6 to 10.0 g/l, and cyanide at 11.0 to 18.5 mg/l. These constituentchanges produced copper deposition rates of 0.0029 to 0.0032 mm/hr. andRHO values of 6.78 to 6.88. The RHO values versus plating rate are shownin FIG. 1 in the two groups resulting from plating rate differences.

EXAMPLE 2

A second autocatalytic copper plating bath was varied as to constituentconcentrations to effect plating rates as follows: formaldehydeconcentration was maintained at 2.6 to 4.1 ml/l, copper at 6.0 to 8.5g/l and cyanide at 7.0 to 14.6 mg/l.

Again, substrates of epoxy-glass fibers with sensitized circuit pathswere immersed for 12-hour periods. The second bath produced platingrates of 0.0021 to 0.0022 mm/hr. RHO values using the four-point probesystem mentioned above were found to be 6.53 to 6.60.

The concentrations were then changed to promote faster plating andformaldehyde was maintained between 3.4 and 4.4 ml/l, copper at 6.0 to8.7 g/l and cyanide at 9.3 to 18.6 mg/l. The plating rates increased andwere from 0.0026 to 0.0028 mm/hr. RHO values at this higher rate rangedfrom 6.75 to 7.00. The RHO values versus plating rates for the secondbath are shown in FIG. 2.

Plated copper deposited in the foregoing examples had a constant bright,shiny appearance irrespective of the deposition rates. There were nochanges in ductility or substrate adhesion as a result of usingdifferent plating rates. From all test data and observations, the purityof the plated metal improved and other properties and qualities of theplating and of the metal remained constant throughout the faster platingrates of the exemplary baths.

Bath plating rates depend primarily on concentrations of reducing agentsused, but manipulation of other bath parameters can also affect theplating rate. In the above examples, formaldehyde was the primary agent.Other baths may use sodium borohydride or another such agent orcombination. The particular proportions must be limited to those levelsthat will not produce bath decomposition.

It will be seen from the foregoing description and test results that anincrease in deposition rate of metal from an autocatalytic bath producesthe very desirable reaction of lower residual resistivity. Thischaracteristic is extremely desirable because of lower energy losses dueto resistance and less circuit fabrication time. Plated metal withspecific residual resistivities can be achieved through rate variations.Thus, by first determining the relationship between residual resistivityand variations in plating rate, a particular resistivity can be obtainedby merely controlling the plating rate of a bath for depositing a metal.This obviously permits the desired residual resistivity profile of aconductor to be varied over its cross section and over its length. Thedisclosed technique of resistance variation now enables some controlover the resistance of plated metal. Heretofore a change in theresistance of copper, for example, could be achieved only throughannealing, a process clearly not suitable for printed circuitfabrication.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that changes in form and details may be madetherein without departing from the spirit and scope of the invention.

What is claimed is:
 1. A metal formed in accordance with a methodcomprising the steps of:determining the variable relationship betweenthe magnitude of a residual resistivity of a metal whenautocatalytically plated onto a substrate and the rate of plating saidmetal; and autocatalytically plating said metal at least three distinct,selected rates of deposition to produce a desired, non-uniform residualresistivity profile over the cross section of the metal, which profileincludes at least three distinct, corresponding residual resistivities.2. The product of claim 1 wherein said plated metal is upper.
 3. A metalformed in accordance with a method comprising the steps of:determiningthe variable relationship between a purity of a metal when electrolesslyplated onto a substrate and the rate of plating said metal; andelectrolessly plating said metal at least three distinct, selected ratesof deposition to produce a desired, non-uniform purity over the crosssection of the plated metal, which profile includes at least threedistinct, corresponding levels of purity.
 4. The metal as deposited inaccordance with claim 3, wherein the metal is copper.